An on-board antenna device comprises: a radiation element (302) formed on an inner-surface of a widow glass (51) for a vehicle (50); a base plate (305) fixed on the inner-surface of the widow glass (51) so as to surround the radiation element (302); a circuit board (308) having a conductive layer in a surface (308a) thereof, which is opposed to the radiation element (302), and a component mounting surface electrically connected to the radiation element (302) in the other surface (308b) thereof; and a housing (312) assembled onto the base plate (305), and the circuit board (308) being contained therein; wherein the circuit board (308) has a cutout portion (308f) to incorporate a connector (324) for a transmission line connection.